Convo on Deco Gas Use and Deco Stops

[Mr T.]

Check out the URL from D-net.

http://www.diverforum.co.uk/cgi-bin/...aO=1&iWz=0

Should be interesting to see how this one rounds off.

[Mark Chase]

Makes a change from getting his teeth into me. Nice one Scott

I have added a bit on the end so maby he will have a go >

Mark Chase

[ralphy]

Fear not my young apprentice - his powers are useless here!

[Stu Sheridan]

Hi guys, guess I got more than I bargained for when I posted that. Apart from my glaring error about using 80% (what a f***wit),there was some good debate, but can anyone tell me more about this "Bakers Dozen"?
Also what are your opinions about the free deco software available. I like GAP and Vplan but found DDlan a bit less user freindly. Any thoughts...?
Stu.

[Bob Cooper]

Stu

Here is the so called "bakers dozen" reasons why not to use 80/20:

I particularly like #13

1) This gas was introduced in an effort to overcome the inability of unqualified student "tech" divers to control their buoyancy in open water, and is as such is yet one more concession to doing things in a convoluted fashion to offset a self- inflicted set of problems brought on by the "doing it wrong" thinking that pervades diving today.

2) A heavy sea is not a problem for a deco stop if it is not posing a lung-loading problem. Look at your depth gauge in a heavy sea and "see" for yourself what the changes are - insignificant, and if they are not, you should either not have been diving or incurring a decompression liability of this magnitude in the first place. In the event of a change in conditions during the dive, see below where the 80/20 becomes a liability rather than an asset.

3) In the interest of using a standardized set of gases for which you can permanently mark your bottles , it is a poor concession to inability to sacrifice the benefits of pure O2 to accommodate a real or perceived lack of skill - learn to dive before taking up tech diving.

4) In this same interest you will find that when you graduate to real diving, as in caves, you will not want to accelerate your PPO2 at lower depths while still being faced with a long decompression at shallower depths, and making bizarre mixes to do this is a dangerous mistake (just like the fantasy of holding an accelerated PPO2 on a rebreather throughout a deco). I am anticipating the thinking that the 80/20 crowd would then go to an additional oxygen in cave without accounting for total exposure, and subject themselves to the risk of tox in the final deco steps. Tox you do not get out of - bends you do.

5) The 80/20 mix is in fact totally useless and contraindicated as a deco gas. At thirty feet it is only a 1.52 PPO2 ( the real 1.6 PPO2 gas would be 84/16) and as such does not either provide the right oxygen window, nor does it does it work as well as pure oxygen without an inert gas at any depth. The gas mixing in your lungs has already lowered the effective PPO2 enough to prevent spiking at 20 feet anyway with the use of pure oxygen - in other words, we are dealing with a simplistic misunderstanding here, or "old wives tale" that is typical in diving.

6) If 100% oxygen is a perceived buoyancy control risk at 20 feet, then why is the same PPO2 (intended) not a risk at 30 feet? This shows the total lack of reasonable logic involved in the decision to use this gas, as well as a lack of understanding of the whole picture ( see the rest of this discussion).

7) Along those lines, all we hear is howling about "oxygen cleaning" above 40% mixtures, and dive shop proprietors on here complaining about scuba tanks with oxygen in them being filled in their shops. With a pure oxygen system, the tank only ever gets filled with oxygen from oxygen tanks, not from every dive shop compressor it sees. Again , this shows the total inconsistency of agency thinking, and reveals that the true reason for this gas is to pretend to lower liability for teaching incompetents to dive, which is bull, and to attempt to accrue some inventive accomplishments to the dive agency pundits who themselves prove that they do no real diving by making this recommendation in the first place. This is like the colored regs, the stages on either side, the quick-release buckle, and the poodle jacket: nonsense of the most obvious nature developed through one-dimensional thinking by those whose universe of understanding is not only severely limited, but blinded by the hubris of not being the "inventor" of the techniques that work.

8) Any perceived decompression benefit of using a higher PPO2 at 30 feet with 80/20 is then given back by the lowered PPO2 at 20 feet, not to mention the fact that the presence of the inert gas in the breathing mixture defeats the purpose of using oxygen in the first place (see the Physiology and Medicine of Diving) . The PPO2 of 80/20 at 20 feet is 1.28, not much of an oxygen window, and at 10 feet it is 1.04 - useless for deco. To make matters worse, you can not get out from your 30 foot stop in an emergency (not doing the other stops) on the 80/20 mix without really risking a type 2 hit.

9) This is a dangerous method to achieve a greater total volume of gas for the bad breathers (another obvious reason the gas is in vogue), who should not be incurring these decos, and even that benefit of having more gas is lost since it is breathed at 30 feet, and then has to last for the other stops. The fact is that gas is effectively saved by using the lower deco gas up to this point, relying on the pressure gradient to both achieve the deco and provide a break from high the previous gas's higher PPO2 prior to going to pure oxygen where the spike could be a problem on an extreme exposure without an adequate low PPO2 break (again this shows that the 80% user is a neophyte diver with no real experience or understanding of the true risks of these dives) .

10) The 20-30% longer 30 foot time on the lower PPO2 is not only overcome on the pure oxygen at the next stops, the breaks do not come into play until the initial good dose of pure oxygen has been absorbed, since you are not spiking from a high previous dose without a break that is effectively achieved on the previous gas. These things need to be understood and taught by the agencies, not some superficial convolution that is designed to obfuscate the problem rather than openly acknowledge and deal with it in a responsible fashion.

11) In an emergency situation, getting onto the pure O2 for 20 minutes or so (for long dives something approximating the bottom time or a any decent interval) would give you a real good shot at getting out of the water having missed the rest of your deco and living through it with pain hits only. You have to think these things all the way though, not go for the transparent superficial thinking of those who merely are trying to "make their mark" with some "great" idea they can call their own. The acid test is, as always, is the caliber of the divers who adopt these practices.

12) If there is some problem with your deco or you otherwise develop symptoms and need oxygen either on the surface or back in the water, it is silly to have not had it there all along. 80/20 is a joke for that purpose, unless you have asthma, in which case any accelerated oxygen mix would be a nightmare. This is again part of the "thinking it all the way through" philosophy which is obviously missing from the 80/20 argument.

13) Only a card-carrying stroke would do something like this, and showing up with 80/20 is no different than wearing a sign on your back saying "I am a stroke, and have the papers to prove it". It announces to all the world that you have no clue, kind of like wearing clip-on suspenders or having dog dirt on your shoes.

[Ian W]

</span>

Originally Posted by [b

Quote[/b] ]Here is the so called &quot;bakers dozen&quot; reasons why not to use 80/20:

<span =''>
Thanks for reminding us what an incoherent ramble much of the &quot;Bakers dozen&quot; is. There are perhaps 2 or 3 reasonable, but arguable, points in there; the rest is opinion and bluster, exemplified by point 13.

Ian W

[Mark Chase]

Thats the final confermation then.

I dive a bungied wing
I have my long hose off the left post
I sling my stages one each side
I dont use a spool
I dive with almost any one
I use 85/15 for deco coz I am concerned about holding 6m in the English Chanel in a force 6.

I AM A STROKE

I like the bit where he said if you arive at 6m and find conditions are too rough for you to hold the stop you shouldent have done the dive. WHAT? Do the write that on your grave stone or somthing.

How bout putting it like this: If you arive at the dive site and conditions are a bit rough (ie 50% of UK dives 80% in the winter/spring months)Then all the divers on 100% stay on the boat and all the 85% strokes get to do the dive


Surley you can deco off of any thing. I have seen good divers doing 50m stuff and using 50% all the way up from 21m. You can deco off air if you like it just takes a long time. I did a 45min deco on air after a deep dive once as there was no access to deco gas on the islend. I spent it pootling up and down the reef and was quite happy to do it. I just wouldent do it hanging in green soup in the UK. &nbsp;

Also the point on presure of fills. Getting a good 100% fill is often hard. I like 200bar plus in my 7ltr rich deco tank and often have 250. Getting this with 100% is not easy. OK I come up with 100+ bar of deco gas but if somthing goes wrong on the dive I might need that one day. I also use that 100 bar on the next few shallow dives as a on boat safety tank. I dont see how breathing 85% can be worse for you than not breathing 02 at all after an incident.

But hey what the #### do I know. I am a Stroke

Just to make sure can any one tell me where to get some good split fins?

Mark Chase &nbsp;

[Dominic]

I'm impressed that, despite there being postings from both Mole &amp; Vic, on the subject of Nitrox mixes, there's been no mention of &quot;Just get a rebreather&quot; yet ;)

[Rob Evans]

Dignified silence... :klo:

[Mark Chase]

Sorry Rob your not getting away with that M8

The yellow box squad are Strokes by default. That GI3 bloke (or GIT as I like to think of him) thinks they are the spawn of satin.

Also I dont see many CCR lads doing 100% 02 deco either. They turn pale at the thaught of using all that gas

Let me see 1.4 ppo2 at 6m thats equivilent to, Errrr let me think, Oh yes I know 87% (Te He)

Mark Chase